Re: [WinPcap-users] WinpCap as "proxy" problems

Wed, 22 Dec 2004 00:04:20 -0800

Thx, your advice worked. I installed etherreal and checked the checksum. Well the calcualtion was correct, just bytes were in a wrong order (stupid mistake). Now everything works! ;-))

Hi,

I'm not familiar with vnc, but wrote a checksum routine at one point. Double
check that the checksum is correct,
easy way is to use etherreal which will show you if checksum is invalid.
It's possible that the checksum was inalid and passed through anyway.

If it's not checksum, if vnc is a tunneling tool, then it's possible that
the actual payload length could be zero, but there is some kind of header
information carried as part of the ip or udp/tcp payload. If so, these could
also be affected by modifying the ip addresses.

mcd






----- Original Message -----
From: "Cefur" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Monday, December 20, 2004 3:29 PM
Subject: [WinPcap-users] WinpCap as "proxy" problems


Info:
I am using Snoop component (Delphi) and winpcap 3.1 beta 4.

Problem:
I had done a sniffer which "sniffs" on two separate NIC's and also sends
packets through each other. It works like a bridge. I had to do this for
some project and everything works ok.

Then I have said it to myself that this could work as an sort of proxy. So
I
connected computer1 to the internet on NIC1 and the other computer2 to the
NIC2.

Then I ran a vnc program on the computer2 to see if I can get response
from 
the computer3 on the internet (through the computer1).

So what happens is next:
an initial packet from computer2 comes to NIC2 where I capture this packet
change its source eth and ip to the NIC1 values, recalculate Checksum and
send it forward through NIC1. Then I get the response (packet) on NIC1
from 
the computer3. Then I change its dest. eth and ip to the computer2 NIC
values and send it through NIC2. So far everything works ok. But when
packet
is received by the vnc program it behaves like it is a wrong/no good
packet
and resends the initial packet??? Btw in both packets the data length is
0.

If I run vnc program from computer1 everything works ok (the data length
of
the first two (send/receive) packets is also 0). Can anybody explain why??
Of course the latency is much to high for any good usage but still it was
just a test for fun and I don't know why it won't work. Any ideas,
explanation, etc?

Thx.



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